The Milky Way's buried treasures

By Phil Plait | August 3, 2011 10:00 am

When I was younger, I’d haul my ‘scope out to the driveway and peruse the heavens. Some of my favorite targets were open clusters: loose aggregations of stars that had dozens or even hundreds of members. These stars are all in the family, born by the same cloud of gas and still bound together by their own gravity.

A lot of these objects are big and bright, easily visible in binoculars — those tend to be close to us in space — and others fainter, harder to spot. Most of those latter ones are just farther away, but some are partially obscured by galactic dust, which robs them of light, like a curtain in a window partially blocking the light form outside.

I used to wonder how many open clusters the Milky Way sported. Thousands, I figured, and I also guessed a lot had been cataloged. But how many were behind such thick curtains of dust, hidden from our view? Some estimates are that there as many as 30,000 such groups. And now we’re starting to find them.

[Click to embiggen, or grab the ginormous 2400 x 2100 pixel version.]

That gorgeous shot is from the European Southern Observatory’s Visible and Infrared Survey Telescope for Astronomy (VISTA), a 4.1 meter ‘scope sitting in northern Chile that sports a 67 megapixel camera. It’s optimized to view the skies in infrared light, which can travel through dust clouds, revealing what lies behind. This picture shows 9 of the 96 new clusters detected by a survey that started only last year! And I had to crop it to fit the blog; the hi-res shot shows 30 clusters in total.

That’s pretty amazing. Those clusters are lovely, but as I was reading the news I wondered how they could differentiate stars in the foreground and background from true members of the cluster. I had a sneaky suspicion it had to do with star colors, and after reading the journal paper I was right. The process involved looking at the colors of all the stars in the image, and comparing them to the colors of the stars that are well away from the cluster (assuming the latter are stars not in the cluster). This creates a way of identifying non-cluster stars, so what remains must be cluster members. I imagine there are probably a few stars left over whose membership is questionable, but given the clusters have hundreds or thousands of stars, a handful mixed up here or there shouldn’t hurt the statistics.

One of the other things that pops out of the analysis is how thick the dust is that’s obscuring the clusters. One of them shocked me: it dims the cluster light by 20 magnitudes, which is a factor of 100 million!*

I wonder how bright it would appear if all that dust weren’t in the way?

The ages of the clusters can be estimated as well (massive stars tend to be blue, and explode when still young, so older clusters are redder). Some are as young as 5 million years, quite young in astronomical terms, while others are more than 400 million years old — meaning some of these clusters formed when animals on Earth first ventured out of the oceans and onto land.

What VISTA is revealing is a previously hidden treasure trove of objects, and I imagine astronomers who study such things are having the times of their lives. And with only 2500 or so clusters cataloged — less than 10% of the estimated total! — there are quite a few left to find.

ESO/J. Borissova


* Each magnitude step is a factor of 2.512 in brightness — that means that a 5 mag difference between stars is 2.5125 = 100. So a 20 mag difference is 2.51220 = 100 million.


Related posts:

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Dying beautifully in a crowd
A box of jewels
The warmth of star birth

CATEGORIZED UNDER: Astronomy, Pretty pictures, Top Post
MORE ABOUT: dust, open clusters, VISTA

Comments (14)

  1. Jes

    BEAUTIFUL. Stars are just gorgeous and each cluster is a delight.

  2. Great game of Pelmanism— two of them are the same, just reframed slightly. Unless two of those open clusters are very close neighbors.

  3. Good catch, Lorelei! There’s a lot of overlap in the lower right-hand shot & the middle right shot.

  4. One thing I’ve always been curious about: with clusters like these, and how groups of stars are born out of nebula, what nearby stars are part of our own ‘solar’ family? I’d imagine that if I were to be standing on a planet that was part of one of the above photo’d clusters, that the night sky would be quite spectacular with a lot of very, very bright nearby stars. While we do have a few fairly bright stars in the sky, nothing in viewing the night sky would lead you to believe that our Sun developed as part of a larger, dense grouping of star generation. Feedback on that question?

  5. Baric

    Milky Way has a belt of stars that are different in chemistry and in motion from stars within the galaxy, suggesting they are the remnants of a galactic collision that may have occurred 10 billion years ago.

    The ring, some 120,000 light years across, is shaped like a doughnut with the Milky Way in its center.

    Source:
    http://www.thesciencenews.info

  6. Pete Jackson

    @4 Phil: It would be great to find other stars that formed with the Sun, but it’s going to be very challenging, if ever possible. All groups that can be so identified (like most of the Big Dipper stars) are less than a billion years old, whereas the Sun is almost 5 times older than that. The Sun, and the other stars formed with it, have travelled around the galaxy more than 20 times since their birth. As they slowly drift away from each other, they start to feel slightly different perturbations from galactic structure. So their velocity vectors in the galactic disk, while staying similar for a billion years or so, will gradually diverge. So even if we can eventually measure accurate velocity vectors for all stars in our galaxy, it might be now difficult to say which ones are for our Sun’s siblings.

  7. CB

    Open clusters are great, but my true love is globs.

    Opens look better in a medium sized scope in light-polluted skies in my driveway, though. :)

  8. Messier Tidy Upper

    @ ^ CB : Better even than the Great Hercules Globular, M13, and (if you can see them from where you live) Omega Centauri and 47 Tucanae?

    Great news and great write-up – thanks BA! :-)

    A reminder that there’s so much even in our own Milky Way galaxy that we don’t, can’t, or just find exceedingly hard to observe and know about.

    Could there have been supernovae in or around some of these clusters that we’ve missed?

    Would I be right in thinking / vaguely remembering from somewhere that there are large parts of our Galaxy we cannot see at all? “Zones of Avoidance” hidden by dust clouds and dense starfields and distance and the geometry of our location versus everywhere else? Are these new “buried treasures” actually in or close to those Zones of Avoidance or is that different again?

  9. Nick

    There is a zone of avoidance (ZOA) because of dust, but in the near-infrared, particularly longer wavelengths such as Ks (2.2 micron) the dust has much less affect so the ZOA is much smaller. One of the VISTA projects is to look for periodic variables all the way through the central regions of the galaxy which would be obscured in the optical. With these you can measure the distances to structures within our galaxy.

  10. Nigel Depledge

    The BA said:

    I wondered how they could differentiate stars in the foreground and background from true members of the cluster. I had a sneaky suspicion it had to do with star colors, and after reading the journal paper I was right. The process involved looking at the colors of all the stars in the image, and comparing them to the colors of the stars that are well away from the cluster (assuming the latter are stars not in the cluster). This creates a way of identifying non-cluster stars, so what remains must be cluster members.

    Oh, man, and here was me thinking they might have done it by measuring the stars’ proper motion.

  11. Nigel Depledge

    The BA said:

    One of the other things that pops out of the analysis is how thick the dust is that’s obscuring the clusters. One of them shocked me: it dims the cluster light by 20 magnitudes, which is a factor of 100 million!*

    I wonder how bright it would appear if all that dust weren’t in the way?

    Well, at a wild guess, I’d say 20 magnitudes brighter.

    Or was that a rhetorical question?

  12. Malcolm

    Dust blocks 20 magnitudes of light. Hmmm, anything that dark here on Earth would have some mass. Has any human collected and measured the mass of this ‘matter of darkness’? Or is it a huge assumption that is a small measure?

  13. Nigel Depledge

    @ Malcolm (12) –
    We can get a pretty good idea of the mass of the dust from the influence it has on the motion of nearby stars.

  14. @9. Nick : Thanks for that info – much appreciated. :-)

    @12. Malcolm : Collected – no, measured – yes. ;-)

    There are ways of doing this based on calculating stellar luminosity & comparing it with observed magnitude and using certain features in the stellar spectrum that indicate absorbtion is taking place among other things.

    @10. Nigel Depledge : “Oh, man, and here was me thinking they might have done it by measuring the stars’ proper motion.”

    Proper motion only works out to a few hundred or so light years, a thousand or so max if I recall right. Of course, detecting proper motion was a first key step in getting some sort of handle on the vastness of the universe & showing us that the “fixed stars” are not-so fixed after all. Using proper motion to determine how far away some nearby stars are allowed us to compare them with more distant stars of the same types and calculate their distances too so it plays a slight role here you could argue! :-)

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